Estimated long-term soil erosion

Erosion leads to a loss of valuable top soil for productive purposes and impacts on soil ecosystem health. Erosion also leads to fine sediment in rivers, streams, lakes, and the coastal environment, causing problems for water and coastal ecosystems (such as the seagrass meadows that provide nurseries for our snapper and other fin fisheries).

While erosion is a natural process, it can be accelerated through human activities. As a general rule, soil erosion in the South Island is more likely due to high rainfall and vulnerable, steep, mountainous terrain, while in the North Island it is due to the historical clearance of forest on steep slopes for pastoral agriculture.

Key findings

In 2012, it was estimated that 192 million tonnes of eroded soil entered New Zealand’s rivers each year; of this, an estimated 84 million tonnes (44 percent) was from exotic grassland.

Of South Island regions, West Coast (49 million tonnes/year), Otago (18 million tonnes/year), and Canterbury (17 million tonnes/year) had the highest levels of sediment movement into waterways. In each of these three regions, sediment movement into waterways estimated to be from exotic grassland was: Canterbury (20 percent), Otago (10 percent), and West Coast (5 percent).

Of North Island regions, Gisborne (40 million tonnes/year), Northland (15 million tonnes/year), and Manawatu/Wanganui (13 million tonnes/year) had the highest levels of sediment movement into waterways. In each of these three regions, sediment movement into waterways estimated to be from exotic grassland was: Manawatu/Wanganui (86 percent), Northland (82 percent), and Gisborne (70 percent).

Note: In some regions the average erosion rate from exotic grassland is higher than the total for the region. The total rate for the region captures erosion from all land cover classes.

Definition and methodology

Annual erosion levels are estimated using the New Zealand Empirical Erosion Model (NZEEM) and the Land Cover Database (LCDB) to define the extent of grassland areas, that is, exotic grassland that comprises low-producing grassland, high-producing grassland, and depleted grassland. Note that depleted grassland can include some indigenous grassland species. Long-term soil erosion is the average mass of soil lost per square kilometre per year (tonnes/km2/year) over a period of approximately 10 years. It is extrapolated from long-term measurements of sediment load in rivers. Extrapolation is based on mean annual rainfall, rock type, and land cover as at 2012. The total sediment in rivers (tonnes/year) for a particular region is the sum of all soil erosion over the entire region (Dymond et al, 2010). Regions are defined using 2017 regional council boundaries.

LCDB maps land cover, but not what the land is used for or how it is used. We do not have the data available to accurately estimate the national extent of accelerated erosion from land uses. This means that this measure cannot track the impact of land management (such as stocking rate or riparian planting).

LCDB maps areas of vegetation that are at least one hectare in size from satellite imagery. This means that small or new areas of vegetation may be missed. Work is being done to create vegetation maps that show a finer level of land cover detail.

NZEEM is the most comprehensive model of erosion we have for gaining an understanding of erosion at a national level. However, it will not model the impact of recent localised storm events which can have a significant impact on erosion rates.